1. Field of the Invention
This invention pertains generally to the field of navigation systems such as, but not limited to, aircraft navigation systems.
2. Description of the Related Art
The continuing growth of aviation has placed increasing demands on airspace capacity and emphasizes the need for the best use of the available airspace. These factors, along with the accuracy of modern aviation navigation systems and the requirement for increased operational efficiency in terms of direct routings and track-keeping accuracy, have resulted in the concept of “Required Navigation Performance” (“RNP”) standards—statements of the navigation performance accuracy necessary for operation within a defined airspace. Some of these standards appear in an Advisory Circular (“AC”) published by the United States (“U.S.”) Federal Aviation Administration (“FAA”) and in a Document (“DO”) published by the Radio Technical Commission for Aeronautics (“RTCA”). For example, the FAA has published AC 120-29A entitled “Criteria for Approval of Category I and Category II Weather Minima for Approach,” and the RCTA has published DO-236B entitled “Minimum Aviation System Performance Standards: Required Navigation Performance for Area Navigation.”
A low-visibility instrument approach procedure (“IAP”) that permits a pilot to operate to a lower altitude than permitted by a standard Instrument Landing System (“ILS”) (which is generally 200 feet height above threshold (“HATh”)) without visually identifying the runway environment may require the use of a radio altimeter system to define a missed approach point represented to the pilot as a decision height (“DH”).
In determining the DH, an aviation-governing authority such as the FAA surveys the approach corridor to the runway to determine the DH relative to the runway. In order to achieve lower minima (i.e., lower DH) for IAPs based upon a satellite navigation system such as the Global Positioning System (“GPS”), preparations have begun in the U.S. to implement the Next Generation Air Transport System (“NextGen”), a system designed to reduce the stress currently experienced by the U.S and address the expected growth in aircraft operations forecasted through 2025. At the time of this writing, the lowest minima for GPS-based approaches such as the Localizer Performance with Vertical Guidance (“LPV”) include a DH of 200 feet HATh.
At the current 200 feet DH, a six second time to alert for errors in GPS navigation is adequate, however, as the DH is reduced the time to alert must be reduced as the time available to recover from a navigation error is reduced as the aircraft altitude is reduced. In addition, for RNP of less than 0.3 nautical miles, the aircraft navigation system must be able to maintain accurate navigation for some time period to allow the aircraft to climb to a safe altitude clear of terrain. The aircraft must be equipped to continue to accurately navigate after a loss or detected error in GPS and any detected fault in GPS must not degrade the navigation system's ability to continue to provide position information after the GPS fault.
RNP may include performance and functional requirements indicated by the RNP type. The RNP type defines the total system error (“TSE”) that may be allowed in lateral and longitudinal dimensions within a particular airspace, where the TSE takes into account path definition errors (“PDE”), navigation system errors (“NSE”), and flight technical errors (“FTE”). The RNP type is used to specify navigation requirements for the airspace or a navigation system that provides a specified level of accuracy defined by a lateral area of confined airspace in which an RNP-certified aircraft operates. For example, an RNP 0.3 level requires a normal navigational accuracy of 0.3 nautical miles (“NM”) at least 95 percent of the total flight time; similarly, an RNP 0.1 level requires a normal navigational accuracy of 0.1 NM at least 95 percent of the total flight time. Hence, a lower level of RNP means a greater level of navigational accuracy. With the availability of GPS, future airspace systems may rely heavily on GPS-based navigation to which stringent standards for NSE may be applied. An IAP specifying an RNP level of less than RNP 0.3 or lower than the standard minima at airports not equipped with an ILS may require a method to detect GPS position errors before the 6.2 second time-to-alert standard that currently exists for systems utilizing satellite based augmentation systems such as Wide Area Augmentation System (“WAAS”) or even the shorter 3 second time to alert for typical ground based augmentation systems.
Undetected errors in the data from GPS or other external navigation signal sources may persist during the period between the time of the external navigation signal source in space fault occurrence and the time at which the navigation system is made aware of the fault by the external navigation aids (pre-detection fault period). In addition, a system may be required to maintain precise navigation for some time after loss of GPS or other external navigational signal source. An aircraft navigation system may utilize the external navigation signal source to help it correct errors in its inertial system, such as inertial sensor biases or velocity errors in its strapdown navigation solution. In this case, if data from the external navigation signal source is used during the pre-detection fault period, it may make erroneous estimates of the inertial system errors. Any erroneous estimates of the inertial system errors could, in turn, cause the navigation system errors to continue to increase at a significant rate even after the navigation system has stopped using the erroneous data from the external navigation signal source.